In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. A phase-change inkjet printer employs phase change inks that are solid at ambient temperature, but transition to a liquid phase at an elevated temperature. The melted ink can then be ejected onto print media or an image receiving member by a printhead in response to firing signals received from a controller.
In a direct-to-media printer, the printheads eject ink drops directly onto print media, for example, paper sheets or a continuous media web. After ink drops are printed on the print media, the printer moves the print media through a nip formed between two rollers that apply pressure and, optionally, heat to the ink drops and print medium. One roller, typically referred to as a “spreader roller,” contacts the printed side of the print media. The second roller, typically referred to as a “pressure roller,” presses the media against the spreader roller to spread the ink drops and fix the ink to the print media.
Pressure rollers typically include a steel cylindrical core coated by an elastomeric layer. During long substantially continuous printing, the temperature of the pressure roller can become elevated due to contact with the heated spreader roller and the print media, which is also heated prior to spreading. Temperature differences along the axial length of the steel core can cause the pressure roller to expand in some locations along the axis of the core, particularly near the center. Non-uniform expansion of the roller alters the shape of the contact the pressure roller makes with the spreader roller, also known as the “nip profile.” Changes in the nip profile can result in the print media wrinkling while traveling through the nip, particularly in continuous media printers. Wrinkling of the print media through the spreader can result in image defects in the printed product.
Temperature increases in the pressure roller caused by its use to help spread ink on the media also alters the properties of the elastomeric layer on the roller. For example, in some pressure rollers, increased heating can cause a reduction in the modulus of elasticity of the elastomeric layer. A decreased modulus of elasticity can further impact the nip profile, compounding the media wrinkle issues noted above. Therefore, improvements in the temperature resistance and uniformity of pressure rollers are desirable.